Field of the Invention
The present invention relates to a system for evaluating the display quality of a transparent display and method thereof. Particularly, the present invention provides a new standard (or, index, measure, scale) and a system for evaluating how well a transparent display presents display information with the background view (e.g., the scene behind the display/objects viewed through the display) through the display at the same time.
Discussion of the Related Art
Various flat panel display devices providing video information including still pictures, moving pictures and/or animations are developed for overcoming many drawbacks of the cathode ray tube such as its heavy weight and bulk volume. Flat panel display devices include the liquid crystal display device (LCD), the field emission display (FED), the plasma display panel (PDP) and the electroluminance device (EL).
The liquid crystal display and the organic light emitting diode display are representative of flat displays that are widely applied in various appliances including portable devices and/or television sets. However, these flat panel displays are developed just for a display system, but they are not applied to various applications. For example, a transparent display displays video information on its screen when the display is activated and the user can see objects behind the display through the display panel when it is not activated. However, the standard for evaluating the quality of a transparent display is not clear and it is very hard for a user and/or consumer to decide which display is suitable for his or her displaying environment and/or purpose.
In more detail, the transparent display is like see through glass when it is not operated, and presents video information when it is operated. In some instances, when it is activated, the user sees only the video information presented by the transparent display. In other instances, even when the transparent display is operated, the user may see the video information and the background scene behind the display panel through the display panel (e.g., a view of the objects behind the display). For example, a heads up display (HUD) is a typical example of such a transparent display. Until now, such a transparent display is only used in environments in which a very expensive display system is applied without regard to the price in the market. However, the requirement of a transparent display in various fields including advertising displays and home appliances is increasing.
FIG. 1 is a perspective diagram illustrating the structure of a transparent display using a liquid crystal display. The transparent liquid crystal display includes a liquid crystal display panel LCDP, a light guide LG and a light source. The liquid crystal display panel LCDP includes two polarization sheets and a liquid crystal panel LCP inserted between the two polarization sheets. The liquid crystal panel LCP includes an upper substrate SU, a lower substrate SL and a liquid crystal layer LC inserted between the two substrates SU and SL.
At the upper side and the lower side of the liquid crystal panel LCP, an upper polarization sheet PU and a lower polarization sheet PL are disposed, respectively. On the inner surfaces of the upper substrate SU and the lower substrate SL, a plurality of lines and black matrixes are disposed as defining a plurality of pixel areas arranged in a matrix manner, and the common electrode and the pixel electrode for driving the liquid crystal layer LC. Further, a color filter for representing a full color picture is included. The upper polarization sheet PU is disposed on the outer surface of the upper substrate SU, and the lower polarization sheet PL is disposed on the outer surface of the lower substrate SL.
Generally, the light axis of the upper polarization sheet PU is perpendicular to the light axis of the lower polarization sheet PL so that a real black level can be reproduced exactly. However, for a transparent display, if the upper polarization sheet PU and the lower polarization sheet PL are disposed like they are in a common liquid crystal display, when it is not activated, light cannot pass through the liquid crystal display panel LCDP. The situation in which the light axis of the upper polarization sheet PU is perpendicular to that of the lower polarization sheet PL is referred to as the ‘Normally Black (or NB) mode,’ because it represents the black level under normal conditions (e.g., while not operating).
For a transparent display, the display can be in a transparent state when the liquid crystal display panel LCDP is not activated. Therefore, it is preferable that the liquid crystal display panel LCDP for the transparent display be made in the ‘Normally White (or NW) mode’ which represents the white level under normal conditions. Unlike the normally black mode, the normally white mode cannot be acquired by the parallel arrangement of the light axes of the upper polarization sheet PU and the lower polarization sheet PL. The polarization characteristics of the liquid crystal layer LC used for the liquid crystal panel LCP should be considered.
For the vertical electric field type of liquid crystal display using a twisted nematic mode liquid crystal layer, even though the light axes of the upper polarization sheet PU and the lower polarization sheet PL are perpendicular, the normally white mode can be acquired. On the contrary, for the horizontal electric field type of liquid crystal display, the normally white mode can be established with the light axes of the upper polarization sheet PU and the lower polarization sheet PL being perpendicular.
Under the liquid crystal display panel LCDP, the light guide LG and the light source LS are disposed. The light source LS is disposed at one side surface of the light guide LG to provide light to the light guide LG. The light guide LG diffuses light from the light source LS throughout the whole inner space of the light guide LG, and refracts light to the upper surface facing the liquid crystal display panel LCDP. To do this, a reflective pattern is disposed on the rear surface (or, bottom surface) of the light guide LG. Since the light guide LG should ensure a transparent condition, the reflective pattern would be one of the prism pattern, the lenticular lens pattern or the micro lens pattern.
As mentioned above, when the transparent display is not activated, it is in a transparent condition like clear glass. On the contrary, when electric power is supplied for using it as a display device, it can provide video information together with the scene behind the display (e.g., objects behind the display remain viewable). Further, the transparent display should include a back light unit that ensures the transparent property, in contrast to a normal liquid crystal display. Therefore, the optical sheets used in normal liquid crystal displays for enhancing the brightness of the back light should not be used for a transparent display.
Hereinafter, referring to FIGS. 2 and 3, a related art transparent display using the organic light emitting diode display will be explained. In particular, FIG. 2 is a plane view illustrating the structure of a transparent organic light emitting diode display, and FIG. 3 is a cross sectional view illustrating the structure of the bottom emission type of transparent organic light emitting diode display along the cutting line I-I′ of FIG. 2. The bottom emission type of transparent organic light emitting diode display according to the related art includes a switching thin film transistor ST, a driving thin film transistor DT connected to the switching thin film transistor ST, and an organic light emitting diode OLE connected to the driving thin film transistor DT.
One pixel area of the transparent organic light emitting diode display includes a light emitting area LEA for representing the video information and a transparent area TRA for penetrating or transmitting/communicating the background scene. For example, the pixel area is defined by a scan line SL, a data line DL and a driving current line VDD, and the pixel area is divided into the light emitting area LEA and the transparent area TRA. Further, the pixel area includes a non-light emitting area where any light for representing video information and from the background scene are not suggested.
The switching thin film transistor ST is formed where the scan line SL and the data line DL cross each other. The switching thin film transistor ST acts for selecting the pixel connected to the switching thin film transistor ST. The switching thin film transistor ST includes a gate electrode SG branching from the scan line SL, a semiconductor channel layer SA overlapping with the gate electrode SG, a source electrode SS and a drain electrode SD. The driving thin film transistor DT drives an anode electrode ANO of the organic light emitting diode OLE disposed at the pixel selected by the switching thin film transistor ST. The driving thin film transistor DT includes a gate electrode DG connected to the drain electrode SD of the switching thin film transistor ST, a semiconductor channel layer DA, a source electrode DS connected to the driving current line VDD, and a drain electrode DD.
The drain electrode DD of the driving thin film transistor DT is connected to the anode electrode ANO of the organic light emitting diode OLE. The organic light emitting layer OL is inserted between the anode electrode ANO and the cathode electrode CAT. Further, the cathode electrode CAT is connected to the base voltage (or, ground voltage) VSS. A storage capacitance Cst is disposed between the gate electrode DG of the driving thin film transistor DT and the driving current line VDD or between the gate electrode DG of the driving thin film transistor DT and the drain electrode DD of the driving thin film transistor DT.
In the view of the cross sectional structure shown in FIG. 3, the gate electrodes SG and DG of the switching thin film transistor ST and the driving thin film transistor DT are formed on the substrate SUB of the transparent organic light emitting diode display. On the gate electrodes SG and DG, the gate insulator GI is deposited. On the gate insulator GI overlapping with the gate electrodes SG and DG, the semiconductor layers SA and DA are formed, respectively. Further, on the semiconductor layer SA and DA, the source electrode SS and DS and the drain electrode SD and DD facing and separated from each other are formed. The drain electrode SD of the switching thin film transistor ST is connected to the gate electrode DG of the driving thin film transistor DT via the drain contact hole DH penetrating the gate insulator GI. In addition, the passivation layer PAS is deposited on the substrate SUB having the switching thin film transistor ST and the driving thin film transistor DT.
In some instances, a color filter CF is further disposed on the passivation layer PAS. In these instances, it is preferable that the color filter CF be formed within the light emitting area LEA. For example, the color filter CF can be formed where the anode electrode ANO would be formed later. For representing full color, the color filter CF may include any one of a red pigment, a green pigment and a blue pigment. The color filter CF set including the red color filter R, the green color filter G and the blue color filter B is arrayed in a matrix manner.
As mentioned above, the substrate SUB having the thin film transistors ST and DT has an uneven surface and level differences because there are many elements. It is preferable for the organic light emitting layer OL to be formed on an even surface to ensure uniform light emission distribution over all of the area of the organic light emitting layer OL. Therefore, in order to make the surface of the substrate SUB smooth, the over coat layer OC (or, the planar layer) is deposited over the substrate SUB.
On the over coat layer OC, an anode electrode ANO of the organic light emitting diode OLE is formed. Here, the anode electrode ANO is connected to the drain electrode DD of the driving thin film transistor DT via the contact hole formed at the over coat layer OC and the passivation layer PAS. It is preferable that the anode electrode ANO is formed within the light emitting area LEA. The ratio of the light emitting area LEA and the transparent area TRA is not strictly defined. That is, it can be selected among the various ratio values according to the specification for the brightness of the display and purpose of the display.
In the bottom emission type of organic light emitting diode display representing full color, the anode electrode includes a transparent conductive material such as an indium tin oxide (or, ITO) or an indium zinc oxide (or, IZO). A bank BN is formed on the substrate SUB having the anode electrode ANO. It is preferable that the bank BN separates the light emitting area LEA and the transparent area TRA and has apertures exposing each area, respectively. If required, the bank BN can have one aperture exposing the light emitting area LEA and the transparent area TRA. Otherwise, the bank BN has a pattern for exposing the light emitting area LEA but not exposing the transparent area TRA. The exposed portion of the anode electrode ANO by the bank BN would be the actual light emitting area.
On the surface of the substrate SUB where the anode electrode ANO of the light emitting area LEA is exposed from the bank BN, the organic light emitting layer OL is formed. For the bottom emission type in which the color filter CF is disposed under the anode electrode ANO, the organic light emitting layer OL may include an organic material which can generate white color. On the organic light emitting layer OL, the cathode electrode CAT is formed. Consequently, the organic light emitting diode OLE including the anode electrode ANO, the organic light emitting layer OL and the cathode electrode CAT and driven by the driving thin film transistor DT is formed.
A transparent display having the structure mentioned above appears as transparent glass when it is not activated so that the background scene can be seen by a user located in front of the display. When the user turns on the display, the user can see the video information with the background scene or without the background scene. This can be applied to various applications.
The transparent display should have a good property for displaying high quality video information and for providing high visual quality of the background scene passing through the display panel. However, the standard (or, index, measure, scale or barometer) for evaluating the visual quality and/or property of a transparent display is not clearly defined in the field market. Until now, the quality of a transparent display has been evaluated by adopting the measurement standards for transparent substrates such as bare glass. For example, the standard for evaluating a transparent substrate according to the related art is measurement of the haze or the clarity.
The haze means the diffusing degrees of the light is defined by the percent of transmitted light that is scattered so that its direction deviates more than a specified angle from the direction of the incident beam (ASTM D 1003). In this test method, the specified angle is 2.5° (0.044 rad). FIG. 4 is a schematic diagram illustrating the method for measuring the haze according to the related art. The light IL radiated from the light source LS enters into the measuring instrument HMD via the entrance ETR after passing through the transparent display TS. Here, the light TL passing through the transparent display TS is scattered. The haze measuring instrument HMD can measure the light out of the exit EXT, where the measured light is within the specified angle)(2.5° about the light incident axis. Using the instrument as shown in FIG. 4, the total amount of the light, Tt, passing through the transparent display TS and measured at the entrance ETR and the partial amount of the light, Td, propagating within the specified angle)(2.5° at the exit EXT are measured, respectively. And then, the haze is calculated by the following Equation 1.
                    Haze        =                                            T              d                        Tt                    ×          100          ⁢                      (            %            )                                              (                  Equation          ⁢                                          ⁢          1                )            
The clarity is defined as the ability to transmit image-forming light, in correlation with its regular transmittance (ASTM D 1746-03), and can be acquired as measuring the ratio of the amount of light passing within the specific angle 2.5° to the whole amount of light transmitted through the ring pattern with the contrast modulation. FIG. 5 is a schematic diagram illustrating the related art method for measuring the clarity. The light IL is radiated from the light source LS. The radiated light TL is measured using the clarity measuring instrument CMD. Disposing a ring pattern CP at the exit EXT of the clarity measuring instrument CMD, and measuring the amount of the light, the clarity can be calculated. As shown in FIG. 5, the light amount, IR, passing through the ring pattern RS and the light amount, Ic, passing through the center circle pattern CS are measured and then the clarity is calculated by the following Equation 2.
                    Clarity        =                                                            I                C                            -                              I                R                                                                    I                C                            +                              I                R                                              ×          100          ⁢                      (            %            )                                              (                  Equation          ⁢                                          ⁢          2                )            
However, these values cannot accurately evaluate the quality of a transparent display exactly. For example, a transparent display having a high value of the haze or the clarity has a worse quality than a transparent display having a lower value of the haze or the clarity.
As mentioned above, at least on one surface of the transparent display (for ensuring the transparent property), various elements configuring the pixel are formed, even though they are not easily seen. Therefore, when light from the background scene passes through the transparent display panel, the light has various optical effects due to the different elements. For example, the incident light from the rear surface of the transparent display may be refracted, reflected and/or absorbed as the light passes through the transparent display panel. Further, because the various elements have tiny patterns, these patterns act as silts so that various optical phenomena such as diffraction and/or scattering occur.
Unlike a transparent liquid crystal display, a transparent organic light emitting diode display has no transparent electrode in the transparent area TRA so that the background light may not be diffracted and/or refracted by the elements. As a result, it has a better transparent quality than a liquid crystal display. However, there are lines disposed between the transparent areas TRA. Especially, for a high resolution transparent organic light emitting diode display, the background light may be easily diffracted and/or refracted by the display elements so that the transparent quality is degraded.
The haze and the clarity are the evaluation standards for bare glass. Therefore, they are not suitable for evaluating the transparent property of a transparent display panel in which various transparent elements are disposed thereon. Consequently, any related standard cannot evaluate the quality of a transparent display exactly or correctly, and there is no method for evaluating the transparent display quality.